Greyhound now lets passengers pay $5 each way for priority seating on bus rides departing from more than 30 cities. You can reserve a specific seat from 30 days in advance (and up to 45 minutes before departure). The fee also gives you the right to board before other passengers.
Like many Greyhound services, this is a "work-in-progress." For example, when we just looked this morning, we didn't see any mention of this service on Greyhound.com, even though it's been in a trial phase since mid-November. (Sigh.) The press release suggests that you see a Greyhound ticket agent for details.